Method for the generation of smoke for use in smoke-curing of foods

ABSTRACT

The invention concerns a method and an equipment for the generation of smoke for smoke-curing of goods. The smoke is generated by means of the top-burning technique, the smoke being recovered only when there is a layer of coals that has been produced from the burning of the fuel and that has a thickness of about 0.5 to 100 mm above the layer of fuel. The layer of coals acts as a regulator of the flow of the combustion air, whereby the burning can be made to take place at a temperature favorable in view of the formation of flavor agents and detrimental substances. The layer of coals also acts as a selective filter as it removes detrimental substances out of the smoke.

The present invention concerns a method and an equipment for thegeneration of smoke for use in smoke-curing of foods.

Smoke-curing is a method for improving the preservation and taste offoods, such as meat and fish. However, it has been noticed that it is adrawback of this process of treatment of foods that, when wood is burntor heated for the generation of smoke, besides the desirable flavoragents detrimental substances are also produced, some of which have beennoticed to be carcinogenic.

The problem caused by the detrimental substances has been realized for arelatively long time, and several modes of operation have been suggestedfor its elimination. As a rule, these prior-art methods are based ontreatment of the smoke formed, by means of which said methods attemptsare made to separate the flavor agents, on one hand, and the detrimentalsubstances, on the other hand, out of the smoke to make fractions oftheir own.

One of these prior-art methods is based on cooling faces placed in thepath of flow of the smoke, the detrimental substances contained in thesmoke being supposed to be condensed on said faces. Such methods alsooperate in the desired way partially, for detrimental substances, suchas tar, remain on the condensing faces out of the smoke. Otherdetrimental substances are also condensed, but the efficiency of themethod is, however, deficient, for only part of the detrimentalsubstances are removed and, on the other hand, desirable flavor agentsare also lost, whereby, in order that the desired flavor could beobtained, the smoke-curing must be more intensive accordingly.

More advanced methods are based on the observation that the flavoragents contained in the smoke are, as a rule, soluble in water and thatthe detrimental substances are correspondingly, as a rule, insoluble inwater. This observation has permitted a sort of "indirect" smoking. Thedesirable flavor agents have been separated from the smoke by means ofwater-washing. This water fraction has then been processed further bymeans of various physical and/or chemical methods so as to obtain aliquid smoke fraction as harmless as possible but rich in flavor. Forthe smoking effect, the foods to be smoke-cured have been soaked in saidliquid smoke fraction to provide them with the flavor. Hereinafter theproducts may still be subjected to a gentle smoking in the traditionalway so as to obtain the desired appearance. It can be considered that,as a result, a product is obtained that is, at least in principle, ofsubstantially higher purity but that has been subjected to a complicatedprocess of treatment.

Correspondingly, methods are also known in which the smoke fractionseparated from the smoke by means of water-washing is processed farenough so that a dry product is obtained from it. This can then be usedin the way of a spice to "smoke" a food. These dry methods may also beassociated with gentle smoking in the conventional way to improve theappearance of the product.

In more recent research related to smoke-curing, attempts have been madeto find solutions for the problem formed by the detrimental substancesby going into the process of formation of smoke itself and to find meansby which the formation of detrimental substances could already beeliminated while, however, not losing the flavor agents. In thesestudies it has been noticed that one factor essential in view of thecomposition of the smoke produced on burning of wood is the temperatureof burning. It has been noticed that the optimum range consists ofburning temperatures slightly below 700° C., i.e. about 650° C. to 700°C. Even in such a case, the formation of detrimental substances cannotbe avoided completely, but it has been noticed that, within theburning-temperature range concerned, the ratio between flavor agents anddetrimental substances is at the optimum. Said ratio of flavor agentsand detrimental substances deteriorates steeply at temperatures above700° C,, whereas at temperatures below 700° C. the deterioration of theratio is less steep. (Potthast: Advances in Food Research, Vol. 29,1984).

An essential factor in keeping the process of burning within saidoptimum range is correct dosage of the combustion air. In addition tothe fact that the oxygen in air acts upon the combustion process proper,air is also directly involved in the secondary reactions after thecombustion process, which said reactions have their role for theultimate composition of the smoke gas.

This most recent approach for the reduction of the problem ofdetrimental substances is to be considered correct, and the advantagesobtained by its means can be supplemented by means of older prior-artmethods, such as condensation of smoke.

As a result of experiments carried out, it has been ascertained that thecombustion process aimed at, and the processes following directly afterit, are associated with circumstances whose correct control has anessential effect on the composition of the smoke obtained, as a lowconcentration of detrimental substances, on one hand, and as a highconcentration of flavor agents, on the other hand.

It has been noticed that a starting point for an achievement offavorable results is the correct burning technique. In experiments ithas come out that, in order that adequate results could be obtained, thegeneration of smoke must be based on so-called top-burning technique,i.e. on the use of a fire chamber in which an essential proportion ofthe combustion air is passed to the fuel through a burnt material layerformed on the top of the burning layer. Even though the use of thetop-burning technique for the generation of smoke for smoke-curing canbe considered as in itself known e.g., from the German Patent No.867,947, this technique is associated with many circumstances by meansof which the quality of the smoke can be affected and which have notbeen explained, e.g., in said publication.

The effect of the layer of coals on the burning process can beconsidered indirect, for when the topburning technique is used, the airentering into the burning zone must pass through the layer of coalsformed on the surface of the fuel, in counterflow with the smoke that isleaving the burning zone. Thus, the layer of coals forms a flow limiterfor the burning air, which said limiter has a direct effect on theintensity of burning, which has again its effect on the temperature ofburning, and thereby on the composition of the smoke formed.

On the other hand, the layer of coals produced acts upon the smoke gasesby means of its considerable absorption effect. It has been noticed thatcoal retains components from the smoke selectively, the selecting beingobviously based on the molecular weights and polarity of the substances.According to the experiments, the selecting, however, takes place in therespect favorably that it has been noticed that compounds found to bedetrimental, such as, e.g., benzo-a-pyrene, adhere to coal more reliablythan phenol compounds to be classified as aromatic compounds.

In respect of these two processes, as regards the layer of coals formedon the surface of the fuel, the thickness of the layer has proved anessential factor, which said factor has obviously a complex effect inthe overall process of generation of smoke.

Firstly, if the effect of the layer of coals is examined in view of theflow or the combustion air, it is clear that the effect of retarding theflow is increased with an increase in the layer of coals. When the fuelis kindled, there is no layer of coals, whereby there is no obstacle forthe passage of the combustion air into the burning zone. At this stage,in view of the formation of smoke, the burning takes place wildly, i.e.mainly by blazing, whereby the burning temperature tends to becomeexcessively high in view of the formation of the various compounds. Inthe smoke, a low level of flavor agents but an abundance of detrimentalsubstances are produced. However, if the fuel and its particle size havebeen chosen correctly, which matter will be returned to later, theburning is evened relatively soon after kindling as a layer of coalsstarts being formed on the surface of the fuel.

Thus, in view of burning, it can be considered that an operative layerof coals has a certain minimum thickness in order that it should have asufficient quieting effect on the wildness of burning. According toexperiments, it has been noticed that this minimum thickness is about0.5 mm, to some extent depending on the degree of coarseness of the fuelused. In order that the desired combustion process could be achieved, acoarse fuel may require that the layer of coals has a minimum thicknessof about 2 mm. After the said layer of coals has been formed, theburning is evened and open flames disappear, whereby the burning goes onby smoldering. In visual observation, at this stage, it can also benoticed that the generation of smoke has increased clearly. On the basisof measurements carried out, it has also been possible to ascertain thatat this stage the temperature in the burning zone is set within therange optimal in view of the composition of the smoke, i.e. attemperatures below 700° C., whereat, firstly, flavor agents are formedin a favorable ratio to the detrimental substances and, on the otherhand, this ratio is not so strongly dependent on changes in temperature.

Likewise, it can be considered obvious that the thickness of the layerof coals has a clear effect on the efficiency of absorption of thelayer. With an increased thickness of the layer the efficiency ofabsorption is also increased.

On the basis of experiments, it has, however, been concluded that thethickness of the layer of coals also has a certain maximum value in viewof the desired effect. It has been noticed that, when the burningproceeds and when the layer of coals is thereby increased, thegeneration of smoke is reduced. Likewise, even based on senseperception, changes can be noticed in the composition of the smoke,which points at essential changes in the combustion process, andpossibly also in the processes after the burning.

In view of the burning itself, it is obvious that, when the layer ofcoals increases, it constitutes such a great limiting factor for theflow of combustion air that the burning zone does not receive asufficient amount of air to maintain burning in the desired way. Theburning is retarded, and likewise the combustion temperature becomeslower. Thereat the amount of smoke obtained is restricted and, on theother hand, the composition of the smoke is changed in an unfavorabledirection because of the excessively low burning temperature, i.e. theamount of flavor agents in relation to the amount of detrimentalsubstances is lowered.

On the other hand, the thickness of the layer of coals can also beexamined from the point of view of absorption. In view of absorption, inthe efficiency of the layer of coals, a certain upper limit is alsoreached, after which a thicker layer of coals no longer increases theabsorption of detrimental substances out of the smoke gases to asignificant extent. The coal particles in the upper layers are filledand lose their efficiency. Thus, the absorption is limited only to acertain limit above the burning layer.

On the contrary, an excessively thick layer of coals may also havedetrimental effects on the efficiency of cleaning of the smoke gases.The layer of coals forms quite an efficient reaction fact for thecomponents in the smoke gases, for reactions with each other, on onehand, and for reactions with the oxygen of the air that flows throughthe layer of coals, on the other hand. As results of these reactions,substances essentially more harmful than those originally present in thesmoke gas may also be formed and be liberated into the smoke gas. Thisafterreaction situation may also be affected essentially by thecircumstance that the to-most coal layers in the filtering layer maystart glowing in the flow of combustion air arriving onto them, in whichcase it is possible that an essential proportion of the flavor agents inthe smoke is destroyed.

On the basis of experiments, the maximum thickness of a layer of coalsplaced on the burning zone has been estimated to be about 100 mm eventhough the fuel particle size may set the favorable limit of maximumthickness clearly lower, at about 20 to 50 mm. The latter value has beenderived at, e.g.. with a fuel with excellent burning properties, themajor part of whose particles were within the range of 125 to 2000 μm.With a clearly coarser fuel the maximum thickness of the layer of coalsmay also be somewhat higher than the above 100 mm.

Based on the circumstances stated above, a method for the generation ofsmoke for smoke-curing of foods has been provided, wherein the smoke isgenerated by burning wood or any other material charring substantiallyin a corresponding way by means of the top-burning technique and whichsaid method is characterized in that the smoke formed during burning ispassed to smoke-curing only when there is a layer of coals produced fromthe burning of the fuel and having a thickness of about 0.5 to 100 mmabove the burning zone. Particularly advantageously, the smoke isrecovered only when the thickness of the layer of coals is within therange of 2 to 50 mm.

The circumstance has also proved quite significant that the layer ofcoals formed during burning should be retained undisturbed during thestage of generation of the smoke to be utilized.

The method of the invention can be carried out either batchwise, whichmode of operation is favorable for small-scale smoke-curing equipments.Of this batchwise mode of operation, it is also possible to provide asubstantially continuous embodiment for smoke-curing equipments ofindustrial scale. Such a continuous embodiment, which, however, operatesbatchwise, is shown in the accompanying FIG. 1, which will be describedin more detail below. A process fully continuous in respect of theprocess of burning can also be accomplished, but the stage of limitingthe layer of coals is associated with processes whose taking intoaccount will be described below in connection with the- description ofan equipment in accordance with FIG. 2.

The process of construction of the equipment to be used in the method isalso associated with general circumstances which should be taken intoaccount in the designing of the equipment. Firstly, it should be notedthat if the fire chamber is made of a thermally highly conductivematerial, such as metal, the burning takes place more efficiently in themiddle of the fire chamber than at the sides, i.e. obviously the loss ofheat through the walls of the fire chamber has an effect on the processof burning. The phenomenon is accentuated if the horizontalcross-sectional form of the fire chamber is angular, in which case theburning takes place at the lowest rate in the corners of the chamber. Anuneven prograss of the fire front in the vertical direction can beconsidered a detrimental facto- in view of the wood consumption and ofthe result, so that this matter ought to be taken into account in theplanning of the device to be used. It would be advantageous that atleast the vertical walls of the fire chamber are made of a material oflow thermal conductivity, e.g of a ceramic material, or it would beadvisable to provide them with thermal insulation. The problem of unevenburning can also be reduced by means of a little lateral flow ofcombustion air passed through the walls of the fire chamber, but thisflow must not be so large that the process of generation of smoke isdisturbed. A cautious heating of the wall of the fire chamber fromoutside also promotes uniform burning.

The problem of uneven burning can also be reduced by choosing the shapeof the fire chamber such that its horizontal section is circular.

In the generation of large quantities of smoke, in the dimensioning ofthe equipment, the fact should also be taken into account that when thearea of burning increases, owing to the opposite directions of flow ofthe combustion air and the smoke gases, flow-technical problems mayarise, which disturb the carrying out of the process, unless the supplyof air to the surface of the fuel has been secured adequately.

In the following Example 1, an experiment is described wherein theprocess of burning was examined by observing the burning temperature ina hill of fuel.

EXAMPLE 1

In the experiment, a smoke oven manufactured for housed use was used,whose outer dimensions were: width 400 mm, depth 450 mm, and height 700mm. In the top portion of the smoke-curing space there was a short fluefor the removal of the smoke. In the bottom part of the smoke oven, afire chamber was provided, into which the fire box containing the fuelcould be inserted through an opening provided in the wall of the ovenand closable by means of a door. The fire box was open at the top buthad closed walls. The cross-section dimensions of the box were 130×140mm. The air needed for burning was passed into the fire chamber throughthe insertion opening for the fire box.

In the experiment, the fuel that was placed into the fire box consistedof sawdust sawn from beech. The particle size of the sawdust was asfollows:

    ______________________________________                                        Particle size (μm)                                                                          %                                                            ______________________________________                                        4000 to 2000     1.1                                                          2000 to 1000     9.4                                                          1000 to 500      27.3                                                         500 to 250       34.4                                                         250 to 125       22.0                                                         125 to 74        4.6                                                          <74              1.2                                                          ______________________________________                                    

The density of said sawdust was about 270 kg/m³, and its moisturecontent out of the wet weight was 7.2%. In the burning experiment, 200 gof sawdust was used (moist weight), which was poured into the fire boxto make a hill. The height of the hill at the center was about 86 mm.

For the observation of the temperature, temperature detectors (NiCr-Nithermoelement, type K, wire length 0.25 mm) had been placed at thecenter of the box at different levels. The heights of the detectors fromthe bottom of the box were:

    ______________________________________                                        Detector    Height from bottom, mm                                            ______________________________________                                        1           85                                                                2           69                                                                3           51                                                                4           40                                                                5           24                                                                ______________________________________                                    

At the beginning of the experiment, the sawdust hill was kindled overall of its face by means of a blow torch. After the open fire haddisappeared from the surface of the hill, the box was placed into thefire chamber of the smoke oven. Adequate combustion air was supplied forburning through the box insertion door by means of natural draught.

In the experiment, the development of the temperature at the differentmeasurement points was observed as a function of time as the smolderingfront made progress in the sawdust.

The results of the experiment are given in the following Graph 1:

From the results it can be seen that after development of a smolderingburning proper (measurement point 2), the temperature in the burningzone rose to a value slightly above 500° C., and in the followingmeasurement points to a somewhat higher level. The peak value in point 4was reached in about 80 min. However, at this stage the generation ofsmoke already started decreasing, and decreased further even though thetemperature at point was still rising, though at a slower rate. It canbe considered that an obvious reason for the retardation was the flowresistance produced by the layer of coals formed on the surface of thehill, which retarded the penetration of the combustion air into theburning layer.

By means of said device and by means of the burning technique describedabove, test smoke-curings were also carried out, wherein fish wassmoke-cured. Out of the smoke-cured products, the content ofbenzo-a-pyrene was determined, which is indicative of detrimentalsubstances. Several tests were carried out by varying the smoke-curingconditions to some extent, i.e. by choosing the burning stage at whichthe fire box was pushed into the smoke oven. In most of the samples theconcentration of benzo-a-pyrene was below the detecting limit, whereasthe highest concentration was 0.05 μg/kg.

For the sake of comparison, a smoke-curing test was also carried out inwhich the fuel burnt in freely flowing air. In this smoke-curing thebenzo-a-pyrene concentration in the product was measured as 0.5 μg/kg.

The equipments in accordance with the invention will be described withthe aid of the accompanying drawing, wherein

FIG. 1 is an illustration of principle of an equipment that operatesbatchwise, and

FIG. 2 shows a second, alternative equipment, likewise as anillustration of principle.

FIG. 1 is a schematical illustration of an equipment for the generationof smoke which operates batchwise, but substantially continuously, forcarrying out the method of the invention.

In the equipment, several smoke generation units 2 are mounted on anendless, horizontal conveyor, such as a chain conveyor. These units areboxes which have solid walls and are open at the top, the dimensions ofsaid boxes being chosen in accordance with the desired generation ofsmoke. The cross-sectional dimensions may be, e.g., about 0.5×0.5 m. Thedimension of height is not critical in any other respect except that itmust be possible to place a sufficient fuel layer in the box. In view ofthe process of generation of smoke, a sufficient dimension of height is,e.g., about 150 mm.

The smoke generation units are arranged to move by means of a stepwisemovement of the conveyor 1 alternatingly via a fuel filling station 3, akindling station 4, a smoke generation station 5, and an emptyingstation 6.

At the filling station 3, a suitable quantity of fuel is measured intothe smoke generation unit from a silo 7, either by means of acompartment dosimeter 8 or by using a leveling doctor, which scrapes anyextra fuel out of the smoke generation unit. As the next stage, thesmoke generation unit proceeds to the kindling station 4. Here thesurface of the fuel is kindled by means of a flame, favorably a gasflame 9, over the entire surface of the smoke generation unit. After thefuel has caught fire all over, the kindling flame is extinguished. Thesmoke generation unit is kept in the kindling station for a suitableperiod of time, during which time the fuel surface burns with an openflame and gradually forms the desired layer of coals on its surface.

The smoke formed during this time is passed by means of a separate flue10 out of the equipment, i.e. this smoke is not used in the smoke-curingprocess. The process of burning of the kindling stage is observed so asto ascertain the formation of an adequate layer of coals. Quite areliable indication of the formation of the desired layer of coals is aclearly increased formation of smoke. As a rule, this stage takes about1 to 10 minutes of time. Thereinafter, as it has been noticed that asufficient layer of coals, i.e. of a thickness of at least 0.5 mm, hasbeen formed, the smoke generation unit can be shifted to the smokegeneration stage 5. Abundant generation of smoke can be noticed in thesmoke generation station, to some extent depending on the particle sizeof the fuel, for a period of about one hour, during which time a layerof coals has been formed on the fuel surface, the thickness of saidlayer of coals being, as a rule, within the range of 20 to 50 mm. Trueenough, in the case of a fuel that is to be classified as coarse,advantageous generation of smoke may, however, go on up to a thicknessof the layer of coals amounting to about 100 mm, or even somewhat more.

After the burning has reached this stage, the smoke generation unit isremoved from the smoke generation station and passed to the emptyingstation 6, wherein the coals and the unburnt fuel are removed.

The process runs through the above cycle batchwise, whereby, at thekindling stage, the situation of burning in the smoke generation stationmust be predicted in order to avoid interruptions in the generation ofsmoke. A deliberately periodic operation is also possible in order tosupply the smoke as subsequent cycles following at certain timeintervals.

The process can be controlled by means of a suitable automatic systemor, alternatively, manually.

If, with the fuel that is being used, it has been noticed that anadvantageous maximum thickness of the layer of coals is, e.g., 50 mm,the smoke generation unit 2 should be filled with fuel at the fillingstage 3 preferably, e.g., up to a layer thickness of about 70 to 80 mm.The layer of fuel that remains on the bottom of the smoke generationunit after the smoke generation stage has a significance of its own asthe layer that receives any tar fractions that may be distilled in thesmoke generation stage.

The combustion air required by the smoke generation stage is passed tothe fuel surface by means of a blower equipment 11, the quantity of airsupplied by said equipment being preferably adjustable. The air must bedosed uniformly over the fuel surface. It is also possible to usenatural draught in the supply of combustion air, which said naturaldraught should also be adjustable.

The smoke generation unit may also include a heat exchanger 12, whichmay be a cooler or a heater. A cooler can be used for condensation ofany impurities that may be contained in the smoke gas. In the case ofhot-smoking, the temperature of the smoke gases can be raised by meansof a heat exchanger.

The ashes are shaken off at the emptying stage of the smoke generationunits into a suitable collecting vessel 13 or onto a removing conveyor.

An alternative equipment for carrying out the invention is shown in theaccompanying FIG. 2. This equipment operates in principle continuouslyeven though its operation involves aspects that require periodization,which will be explained below.

The equipment comprises a fire chamber 15, a mantle 14 that surroundsthe top portion of the fire chamber, means 16 for the supply of the fuelto the bottom portion of the fire chamber, means 17 for the supply ofcombustion air substantially to the mouth of the fire chamber, means 18for passing the smoke into the smoke oven, means 18' and 18" forarranging by-pass flow of smoke, as well as means 19 for the removal ofthe combustion residue out of the equipment.

The mantle 14 of the equipment may have a relatively simpleconstruction, in principle a metal sheet construction, for during theoperation of the equipment it is not subjected to substantial thermalstrains. On the contrary, the fire chamber 15 itself is more demandingin respect of the construction. On the basis of the circumstancesexplained above, it is preferably manufactured as of a circular section,whereat its bottom portion 15' may consist of a metal tube. On thecontrary, in the range 15" of the burning zone proper, the fire chambershould be made thermally insulated, or it may be made of a material ofpoor thermal conductivity, e.g. of a ceramic material.

The fuel is passed to the bottom portion of the fire chamber by means ofa suitable feeding equipment, such as a screw feeder 16. For the supplyof the combustion air to the mouth of the fire chamber, the equipmentmay be provided with a separate pipe system 17, or alternatively thecombustion air may be passed through appropriately dimensioned openingsmade into the mantle 14. Advantageously, the fire chamber should beprovided with temperature detectors 20 placed substantially at the levelof the desired burning zone.

The equipment of the sort concerned can be operated as follows.Appropriately finely divided wood material is fed by means of the screwfeeder 16 into the fire chamber 15 until the fire chamber is full to thebrim. Hereupon the fuel face is kindled all over, and an amount of airsufficient for burning is passed to the fuel. The smoke gas developed atthis stage is passed out of the equipment, e.g. through a branch pipe18' of the flue 18, i.e. the smoke gas of this kindling stage is notutilized. After the burning has been stabilized, which can be noticedquite reliably, e.g., on the basis of the quantity of smoke that isgenerated, the smoke gas is passed to smoke-curing, and the burning isallowed to go on for a certain time (1 to 10 min) estimated in advance,it having been noticed that the fire front proceeds during said periodof time in the direction of depth over said about 30 to 50 mm, in somecases about 100 mm. After this, the flue-gas by-pass duct 18' is openedand the feeder equipment 16 is started momentarily. The new fuelentering into the fire chamber pushes the fuel column upwards, whereatthe layer of ashes in the lateral areas of the fire chamber falls offfrom the mouth of the fire chamber onto the bottom of the mantle 14 andcan be removed by means of the ash-removing means 19.

The period of supply of fresh fuel unavoidably causes a stirring effectin the layer of coals placed on the burning layer, so that during thestirring and during the subsequent period of stabilization of burningthe smoke must be passed out of the smoke-curing equipment through theby-pass duct 18'. After the burning has been stabilized, the smoke canagain be passed to smoke-curing. The procedure is repeated a few times,whereby a suitable hill has been formed at the mouth of the firechamber, the surface layer of said hill being discharged substantiallyall over each time when fresh fuel is supplied. In principle, by meansof the equipment a continuous process of smoke generation is provided,even though the stirring and the subsequent stabilization of the burningcause their breaks in the generation of usable smoke.

Said period of supply of fresh fuel should perhaps preferably becontinued, at least in some periods of supply, long enough so that afresh fuel layer is obtained in the burning equipment up to the surface.In such a case, the tar fraction can also be removed out of theequipment, which said fraction is distilled out of the fuel ahead of theburning front and is concentrated in the fuel placed below. After such aperiod of supply, the hill must be kindled again in the way describedabove.

Differing from the above, the equipment may also be provided with asuitable doctor, which removes the layer of ashes from the mouth of thefire chamber after each period of supply. In such a case, in theutilization of the smoke, the above by-passing should also be applied.

The operation of the equipment can be controlled relatively reliablymanually by observing the generation of smoke and by regulating thesupply of fuel accordingly. The operation can also be made automatic ina relatively simple way by placing temperature detectors 20 in theburning zone and its direct proximity, whereat, on the basis of theinformation supplied by said detectors 20, it is possible to concludethe lengths and frequencies of the required fuel supply periods by meansof a simple logic equipment so as to make the burning zone remain at thedesired distance from the mouth of the fire chamber.

In view of the carrying out of the method and the operation of theequipment, it is also essential that the fuel used is chosen correctly.Usable fuel is, e.g., alder, which is used traditionally forsmoke-curing. Other usable fuels are beech and juniper, and in somecases also peat. Mixtures of the above fuels can, of course, also beused, above all when different flavors are aimed at. As a suitablemoisture content of wood fuel can be defined "air dry". A fuel dryerthan that can also be used, but the burning of a fuel of a highermoisture content may cause difficulties.

It has also been noticed that the granularity of the fuel has its effectin the control of the burning process, and as a general definition forthe granularity it is possible to use the expression sawdust. On thebasis of screening tests, it has been noticed that the major part of theparticles in a well operative fuel are within the range of 125 to 2000μm.

What is claimed is:
 1. A method for smoke-curing a food comprisingburning a finely divided material which generates smoke during charringto form a layer of coals on the surface of the finely divided material;forming a burning zone which progresses through the finely dividedmaterial by passing a combustion air through said layer of coals in acounterflow direction to the flow of smoke generated by the burning ofthe finely divided material; contacting the food to be smoke-cured withthe smoke obtained from the burning finely divided material when saidlayer of coals formed on the surface of the finely divided material hasa thickness of about 0.05 to 100 nm.
 2. The method of claim 1 whereinthe food to be smoke-cured is contacted with the smoke obtained from theburning finely divided material when said layer of coals formed on thesurface of the finely divided material has a thickness of 2 to 50 mm.